1. Field of the Invention
This invention relates broadly to ocular lens devices. More particularly, this invention relates to intraocular lens devices that are surgically implanted within the capsular bag of a human eye.
2. State of the Art
The human eye has an anterior chamber between the cornea and the iris, a posterior chamber behind the iris containing a crystalline lens, a vitreous chamber behind the lens containing vitreous humor, and a retina at the rear of the vitreous chamber. The crystalline lens of a normal human eye has a lens matrix in addition to a lens capsule attached about its periphery to the ciliary muscle of the eye by zonules. This lens capsule has elastic optically clear anterior and posterior membrane-like walls, which are commonly referred by ophthalmologists as anterior and posterior capsules, respectively. Between the iris and ciliary muscle is an annular space called the ciliary sulcus.
The human eye possesses natural accommodation capability. Natural accommodation involves relaxation and constriction of the ciliary muscle by the brain to provide the eye with near and distant vision. This ciliary muscle action is automatic and shapes the natural crystalline lens to the appropriate optical configuration for focusing the light rays entering the eye onto the retina.
The human eye is subject to a variety of disorders which degrade, or totally destroy, the ability of the eye to function properly. One of the more common of these disorders involves progressive clouding of the crystalline lens matrix resulting in the formation of what is referred to as a cataract.
It is common practice to cure a cataract by surgically removing the clouded crystalline lens and implanting an artificial intraocular lens in the eye to replace the natural lens. The prior art is replete with a vast assortment of intraocular lenses. Examples of such lenses are described in the following patents: U.S. Pat. Nos. 4,254,509, 4,298,996, 4,842,601, 4,963,148, 4,994,082, and 5,047,051. As is evident from the above patents, intraocular lenses differ widely in their physical appearance and arrangement.
Typically, cataracts are surgically removed by anterior capsulotomy, which involves forming an incision in the sclera beneath the cornea. Tooling is inserted through this incision and manipulated to open the anterior capsule of the natural lens while leaving intact a capsular bag. This capsular bag has an elastic posterior capsule, an anterior capsular remnant or rim about the anterior capsule opening and a capsular bag sulcus. The capsular bag sulcus is located between the anterior capsule remnant and the outer circumference of the posterior capsule. The capsular bag remains attached about its periphery to the surrounding ciliary muscle of the eye by the zonules of the eye. The lens matrix is extracted from the capsular bag through the scleral incision by phacoemulsification and aspiration (or in some other way). An intraocular lens is then implanted through the scleral incision such that it lies within the capsular bag.
A relatively recent and improved form of anterior capsulotomy known as capsulorhexis forms a generally circular-shaped opening through the anterior capsule by tearing the anterior capsule of the natural lens capsule along a generally circular tear line substantially coaxial with the lens axis and removing the generally circular portion of the anterior capsule surrounded by the tear line. If performed properly, capsulorhexis provides a generally circular opening through the anterior capsule of the natural lens capsule substantially coaxial with the axis of the eye and surrounded circumferentially by a continuous annular remnant or rim of the anterior capsule having a relatively smooth and continuous inner edge bounding the opening.
Another anterior capsulotomy procedure, referred to as an envelope capsulotomy, forms a generally arch-shaped opening through the anterior capsule by cutting a horizontal incision in the anterior capsule, then cutting two vertical incisions in the anterior capsule intersecting and rising from the horizontal incision, and finally tearing the anterior capsule along a tear line having an upper upwardly arching portion which starts at the upper extremity of the vertical incision and continues in a downward vertical portion parallel to the vertical incision which extends downwardly and then across the second vertical incision. This procedure produces a generally arch-shaped anterior capsule opening centered on the axis of the eye. The opening is bounded at its bottom by the horizontal incision, at one vertical side by the vertical incision, at its opposite vertical side by the second vertical incision of the anterior capsule, and at its upper side by the upper arching portion of the capsule tear.
Another capsulotomy procedure, typically referred to as can opener capsulotomy, forms a generally circular-shaped opening through the anterior capsule by piercing the anterior capsule at a number of positions along a circular line substantially coaxial with the axis of the eye and then removing the generally circular portion of the anterior capsule circumferentially surrounded by the line. This procedure produces a generally circular anterior capsule opening substantially coaxial with the axis of the eye and bounded circumferentially by an annular remnant or rim of the anterior capsule.
Intraocular lenses differ widely in their physical appearance and arrangement, yet generally have a central optical region (or optic) and haptics which extend outward from the optic and engage the interior of the eye in such a way as to support the optic in a position centered on the axis of the eye. Intraocular lenses also differ with respect to their accommodation capability, and their placement in the eye. Accommodation is the ability of an intraocular lens to accommodate, that is to focus the eye for near and distant vision. Most non-accommodating lenses have single focus optics, which focus the eye at a certain fixed distance only and require the wearing of eyeglasses to change the focus. Other non-accommodating lenses have bifocal optics which image both near and distant objects on the retina of the eye. The brain selects the appropriate image and suppresses the other image, so that a bifocal intraocular lens provides both near vision and distant vision sight without eyeglasses. Bifocal intraocular lenses, however, suffer from the disadvantage that 20% of the available light is lost in scatter, thereby providing lessened visual acuity. Newer intraocular lenses, such as the intraocular lens described in U.S. Pat. No. 6,685,741, achieve multifocal accommodation in response to compressive forces exerted on the haptics of the lens. Such compressive forces are derived from natural brain-induced contraction and relaxation of the ciliary muscle and increases and decreases in vitreous pressure. Such accommodating intraocular lenses are surgically implanted within the evacuated capsular bag of the patient's eye through the scleral incision and anterior capsule opening in the capsular bag. The haptics of the lens are situated within the outer perimeter of the capsular bag and are designed to support the optics along the optical axis of the eye in a manner that minimizes stretching of the capsular bag.
After surgical implantation of the intraocular lens in the capsular bag of the eye, active endodermal cells on the posterior side of the anterior capsule rim of the capsular bag causes fusion of the rim to the elastic posterior capsule wall by fibrosis. This fibrosis occurs about the haptics of the IOL in such a way that the haptics are effectively “shrink-wrapped” by the fibrous tissue in such a way as to form radial pockets in the fibrous tissue. These pockets contain the haptics with their outer ends positioned within the outer perimeter of the capsular bag. The lens is thereby fixated with the capsular bag with the lens optic aligned with the optical axis of the eye. The anterior capsule rim shrinks during fibrosis, and this shrinkage combined with the shrink-wrapping of the haptics causes some radial compression of the lens in a manner which tends to move the lens optic along the optical axis of the eye. The fibrosed, leather-like anterior capsule rim prevents anterior movement of the optic and urges the optic rearwardly during fibrosis. Accordingly, fibrosis induced movement of the optic occurs posteriorly to a distant vision position in which either (or both) the optic and inner ends of the haptics press rearwardly against the elastic posterior capsule wall, thereby stretching the posterior capsule wall rearwardly.
With time, depending on the rearward pressure of the intraocular lens on the posterior capsule wall as well as other factors (such as lens material, lens geometry, angulation, sharpness, wrinkles in the posterior capsule wall, etc), epithelium cells can migrate between the posterior capsule wall and the lens and reside and multiply in these spaces. Excessive build up of the cells in this area can lead to opacification of the optic. This opacification, commonly referred to as posterior capsule opacification (PCO), causes clouding of vision and can lead to blurring and possibly total vision loss. The process of PCO is slow and clinical changes often take one to two years to become apparent. PCO is typically treated by YAG laser capsulotomy. However, in terms of health economics, PCO is very expensive to treat.
Special care must also be taken that the material of the lens optic maintains transparency after it has been implanted within the eye and subject to ocular fluids. This characteristic is referred to herein as “in vivo transparency” or “in vivo transparent”. Any significant clouding of the lens optic due to its interaction with ocular fluids can lead to blurring and possibly total vision loss in a manner similar to PCO. In U.S. Pat. No. 6,102,939, the inventor describes the crack resistance and biostability of a prosthesis implanted in vivo wherein the prosthesis formed from a polyolefinic copolymer material having a triblock polymer backbone comprising polystyrene-polyisobutylene-polystyrene, which is herein referred to as “SIBS”, while also mentioning the desirability of long term elastomers for use in intraocular lenses as well as a long list of other applications (e.g., vascular grafts, endoluminal grafts, finger joints, indwelling catheters, pacemaker lead insulators, breast implants, hear valves, etc.). However, this patent fails to address important considerations (such as in vivo transparency) with regard to the suitability of the SIBS material for use in the ocular environment.
Modern intraocular lenses are made flexible where they can be folded in half to enable placement in the capsular bag through the scleral incision. However, such lenses typically have relatively lower indices of refraction, and thus such intraocular lenses are required to be thicker to provide the desired magnification characteristics of the intraocular lens. More particularly, the prior art foldable intraocular lenses are typically made of a silicone-based polymer with an index of refraction of 1.3 to 1.4, or made from an acrylic material with an index of refraction of 1.46. Such refractive indices are relatively low (for example, PMMA which is typically used for a rigid IOL has a refractive index of 1.49). Thus, such intraocular lenses are required to be thicker to provide the desired magnification characteristics of the intraocular lens. One skilled in the art understands that the magnification of a lens is dependent upon three values: i) radii of curvature; ii) index of refraction; and iii) thickness of the lens. Thus, for any given lens thickness, a greater index of refraction provides a greater degree of magnification. Alternatively, for any desired magnification, the higher the index of refraction enables the lens to be thinner. The thinner the lens, the smaller it can be folded or rolled. This allows the scleral incision to be made smaller and possibly avoids the use of sutures in closing the scleral incision. Suturing the scleral incision is disadvantageous because the scleral incision site, if not sutured properly, becomes a site of infection and leakage of aqueous fluid. Moreover, if the sutures are too lose or tight, astigmatism can form due to distortion of the cornea. On the other hand, if the scleral incision is small (e.g., less than a 2 mm slit), the incision will close on its own without the use of sutures. In addition, the thinner the lens, the more its radius of curvature can be deformed and/or axial displaced by changing tension in the anterior capsule by muscular contraction, thereby providing for enhanced accommodation after implantation.
Modern intraocular lens also act as a spectral filters that block out ultra-violet (UV) light that may burn the retina. Such UV light blocking capability is typically provided by additives that absorb UV radiation. Such additives are generally molecules that contain aromatic groups. These additives can migrate out of the polymeric material of the lens and cause toxic reactions.
Thus, there remains a need in the art to provide an intraocular lens device that is realized from a flexible biocompatible material that maintains in vivo transparency and has a relatively high index of refraction. Such features provide for a foldable intraocular lens that requires a small incision in the sclera, and thus provides for more effective healing of the eye as noted above. The relatively high index of refraction also provides for an intraocular lens with improved magnification capabilities.
There is also a need in the art to provide an intraocular lens device that is realized from a biocompatible material with ultra-violet light blocking capability that does not risk toxic reaction in the eye.
There is also need in the art to provide an intraocular lens device that inhibits epithelium cell migration and multiplication to thereby protect against PCO.
Several methods of manufacturing flexible, biocompatible intraocular lens devices are known, such as injection molding, spin casting, compression molding and transfer molding. These methods typically employ a polished stainless steel mold cavity that is formed in a geometry that achieves the desired curvature. Several significant problems have been associated with these molding techniques. One problem is that the stainless steel mold requires cleaning between molding cycles, which increases the labor costs and production costs associated with the finished product. Another problem is that steel molds typically have small gaps between the mold halves that allow “flash” to form in the gaps during the molding operation. Flash is unwanted material attached to the mold parting line on the finished product. This flash material must be removed from the finished product, which again increases the labor costs and production costs associated with the finished product.
Thus, there remains a need in the art to provide improved techniques for the manufacture of flexible, biocompatible intraocular lens devices which are less expensive than the prior art manufacturing techniques.